Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this disclosure and are not admitted to be prior art by inclusion in this section.
In a downlink multi-user multiple input multiple output (MU-MIMO) system, a communication node (e.g., a base station) serves one or more user equipments (e.g., other communication nodes, mobile stations, and/or the like) simultaneously through appropriate spatial multiplexing. Thus, in a MU-MIMO system, the base station simultaneously transmits communication signals to one or more user equipments (UEs).
FIG. 1 schematically illustrates a communication system 100. The communication system 100, which is a MU-MIMO communication system, includes a communication node 104 that wirelessly communicates with a plurality of UEs 120, 130, 140 and 160.
The communication node 104 includes a plurality of transmit antennas 106a, . . . , 106d, and a plurality of receive antennas 108a and 108b. The UE 120 includes a plurality of receive antennas 126a and 126b and a transmit antenna 128a. Similarly, the UE 130 includes a plurality of receive antennas 136a and 136b and a transmit antenna 138a, the UE 140 includes a plurality of receive antennas 146a and 146b and a transmit antenna 148a, and the UE 160 includes a plurality of receive antennas 166a and 166b and a transmit antenna 168a. 
Prior to transmitting data signals, the communication node 104 usually transmits pilot or control signals to the UEs 120, . . . , 160 through one or more of the plurality of transmit antennas 106a, . . . , 106d. Based at least in part on signals (e.g., pilot signals) received from the communication node 104, each individual UE 120, 130, 140, and/or 160 estimates a condition of the wireless channels between the communication node 104 and the UE. For example, UE 120 estimates a channel matrix H120, which is representative of quality of a wireless communication channel between the communication node 104 and the UE 120. Similarly, UEs 130, 140 and 160 estimate respective channel matrices H130, H140, and H160 that are representative of quality of respective wireless communication channels.
In the communication system 100, the communication node 104 and/or one or more of the UEs 120, . . . , 160 generally store a common codebook C. Thus, the codebook C is a shared codebook, which is shared between the communication node 104 and/or one or more of the UEs 120, . . . , 160. The codebook C includes a plurality of candidate precoding matrices c1, . . . , cN. That is, C={c1, . . . , cN}.
Based at least in part on the respective estimated channel matrices H120, . . . , H160, each of the UEs 120, . . . , 160 select a respective precoding matrix from the plurality of candidate precoding matrices stored in the codebook C. For example, UE 120 may select precoding matrix c2, UE 130 may select precoding matrix c4, and so on.
Selection of the precoding matrix at an UE is based at least in part on the associated estimated channel matrix. For example, a precoding matrix u1 is selected at UE 120 such that:
                              u          1                =                  arg          ⁢                                    max                                                c                  j                                ∈                C                                      ⁢                                                                                                                        (                                              H                        120                                            )                                        ⁢                                          (                                              c                        j                                            )                                                                                        2                            .                                                          Equation        ⁢                                  ⁢                  (          1          )                    
As discussed later with respect to Equation 4, in a case where cj is the selected precoding matrix, then (H120) (cj) is representative of a signal power component in the data signal received by the UE 120 from the communication node 104. Thus, in Equation 1, out of the N number of candidate precoding matrices c1, . . . , cN included in the codebook C, the selected precoding matrix u1 at UE 120 is a precoding matrix that is associated with a relatively high (e.g., maximum) signal power component in data signal received by the UE 120 from the communication node 104.
Similarly, UE 130, UE 140 and UE 160 select precoding matrices u2, u3 and u4, respectively, from the codebook C based at least in part on respective channel matrices H130, H140, and H160. For example, as previously discussed, UE 120 may select precoding matrix c2 (i.e., u2=c2), UE 130 may select precoding matrix c4 (i.e., u3=c4), and so on.
Each of the UEs 120, . . . , 160 then transmits an index of the respective selected precoding matrix to the communication node 104. This index is also referred to as precoding matrix index (PMI), as this index is representative of the associated selected precoding matrix. That is, each of the UEs 120, . . . , 160 feeds back respective PMIs to the communication node 104. For example, if UE 120 selects precoding matrix c2 (i.e., if u1=c2), then UE 120 feeds back (e.g., through transmit antenna 128a) the PMI corresponding to precoding matrix c2.
The communication node 104 receives (e.g., through one or more of the receive antennas 108a and 108b) the selected PMIs from each of the UEs 120, . . . , 140, and looks up the associated precoding matrices from common codebook C (stored in the communication node 104) using PMIs received from the UEs 120, . . . , 160. The communication node 104 utilizes the looked up precoding matrices for beamforming, while transmitting subsequent signals to the UEs.
As the communication system 100 is a MU-MIMO system, the communication node 104 may serve more than one UE simultaneously through spatial multiplexing. In an example, the communication node 104 transmits signal to UE 120 and UE 130 simultaneously through spatial multiplexing. This may be the case when, for example, the selected precoding matrices u1 and u2 of UE 120 and 130, respectively, are orthogonal to each other. In such a case, signal transmitted by the communication node 104 to the UEs 120 and 130 is given by:x=(u1)(x1)+(u2)(x2),  Equation (2),where x1 and x2 are modulated symbols intended for UEs 120 and 130, respectively, and u1 and u2 are the precoding matrices for UE 120 and UE 130, respectively.
The signal received by, for example, UE 120 (e.g., by the receive antennas 126a and 126b of UE 120) is given by:y120=(H120)x+n,  Equation (3),where n is the white noise at the receive antennas 126a and/or 126b of UE 120, and x is the signal transmitted by the communication node 104 (e.g., see Equation 2). The signal y120 received by UE 120 may be further simplified as:
                                          y            120                    =                                                    (                                  H                  120                                )                            ⁢                              (                                  u                  1                                )                            ⁢                              (                                  x                  1                                )                                      +                                                                                (                                          H                      120                                        )                                    ⁢                                      (                                          u                      2                                        )                                                                    ︸                                      unknown                                          interference                      ⁢                                                                                          ⁢                      direction                                                                                  ⁢                              (                                  x                  2                                )                                      +            n                          ,                            Equation        ⁢                                  ⁢                  (          4          )                    
Symbol x1 is intended for UE 120 and symbol x2 is intended for UE 130. Accordingly, the term (H120)(u1)(x1) is the signal component in the data signal received by the UE 120, and the term (H120)(u2)(x2) is the interference component in the data signal received by the UE 120.
The UE 120 previously selected the precoding matrix u1 and transmitted the PMI associated with precoding matrix u1 to the communication node 104. Accordingly, the UE 120 knows the precoding matrix u1 (e.g., has the precoding matrix u1 stored in the UE 120), and hence, the term ((H120)(u1)) is known to the UE 120. That is, a signal direction (associated with (H120)(u1)) of data signal y120 is known to UE 120.
However, the UE 120 is not aware of the precoding matrix u2 (or the PMI associated with the precoding matrix u2), as the PMI associated with the precoding matrix u2 was transmitted by UE 130 (but not by UE 120) to the communication node 104. Accordingly, an interference direction (associated with (H120)(u2)) of the data signal y120 is unknown to the UE 120.
For purposes of this disclosure and unless otherwise mentioned, a precoding matrix and an associated PMI corresponding to a signal direction of a UE are referred to herein as “signal precoding matrix” and “signal PMI,” respectively, for the UE. For example, the precoding matrix u1 is associated with signal direction for the UE 120. Accordingly, for the UE 120, the precoding matrix u1 is referred to herein as signal precoding matrix, and the associated PMI is referred to herein as signal PMI.
For purposes of this disclosure and unless otherwise mentioned, a precoding matrix and an associated PMI corresponding to an interference direction of a UE are referred to herein as “interference precoding matrix” and “interference PMI,” respectively, for the UE. For example, the precoding matrix u2 is associated with interference direction for the UE 120. Accordingly, for the UE 120, the precoding matrix u2 is referred to herein as interference precoding matrix, and the associated PMI is referred to herein as interference PMI. Also, the interference precoding matrix of one UE is the signal precoding matrix of another UE. For example, the interference precoding matrix u2 of UE 120 is the signal precoding matrix of UE 130.
Referring again to Equation 4, the UE 120 knows the respective signal precoding matrix, the signal PMI, and the signal direction of data signal received by the UE 120. However, the UE 120 is unaware of UE 120's interference precoding matrix, interference PMI, and the interference direction.
As the interference direction is unknown to UE 120, the UE 120 in the conventional communication system 100 of FIG. 1 ignores the interference and performs, for example, maximum ratio combining (MRC) for data detection, which is limited at high signal-to-noise ratio (SNR). Various systems have overcome this by feeding forward, to the UE 120, the interference information (e.g., interference PMI) so that minimal mean square error (MMSE) receive combining is used instead of MRC. However, this increases the overhead and may not be feasible with higher order systems where the number of interfering streams may be more than one.